Syrian troops fired on thousands of protesters who spilled out of mosques after midday prayers Friday , and state media reported that a roadside bomb had killed 10 soldiers, as the latest diplomatic efforts failed to halt more than 13 months of violence.

The UN hopes to have 30 ceasefire monitors in Syria next week and plans are being made for the deployment of up to 300, although Syria is still refusing to accept demands that observers be able to use their own helicopters and planes to visit trouble spots. The UN is also trying to ramp up its humanitarian response and send more food, medicine and aid workers to Syria, said John Ging, the head of emergency response at its Office for the Co-ordination of Humanitarian Affairs.

"The whole infrastructure of the country is under strain," Ging said. He added that the Syrian regime had finally acknowledged that there was a "serious humanitarian need" and that this should ease the aid mission.

The UN estimates some 230,000 Syrians have been displaced and more than 9,000 killed since the uprising against President Bashar Assad erupted more than a year ago. The revolt began with largely peaceful protests, but has grown increasingly violent as the opposition has taken up arms in response to a brutal regime crackdown.

A UN-brokered ceasefire that technically went into effect last week has been steadily unravelling, with regime forces continuing to shell rebel-held neighbourhoods in the central city of Homs and opposition fighters ambushing government troops. The truce is still seen as the most viable way to end the bloodshed, simply for a lack of other options.

Western powers have called for Assad's to step down, but the Syrian leader has dug in, unleashing his military on an ill-equipped and fractured opposition, and there appears to be little appetite in the international community to try to dislodge him by force with an operation similar to the one that helped topple Libyan leader Muammar Gaddafi last year.

Instead, the US secretary of state Hillary Clinton called on Thursday for the UN security council to adopt an arms embargo and other tough measures against Syria. The UN secretary general, Ban Ki-moon, took a hard line against Damascus, saying Syria was not honouring the ceasefire and that violence was escalating.

As part of the truce, Assad was to withdraw troops and tanks from urban centres and allow peaceful anti-regime marches, which the opposition has staged every Friday since the uprising began. He has ignored both provisions and continued attacking opposition strongholds, though the overall level of violence is down compared with the period before the truce.

On Friday, protests were reported in the capital, Damascus, and its suburbs, as well as in the northern city of Aleppo, the central regions of Hama and Homs, in eastern towns near the border with Iraq and in the southern province of Daraa. Demonstrators spilled out from mosques onto the streets, calling for Assad's downfall and chanting in support of the country's rebel forces, activists said.

"Security is extremely tight in Damascus," said activist Maath al-Shami, adding that, despite the heavy presence of plain-clothes security agents, there were protests in the capital's Qaboun, Midan, Barzeh and Mazzeh neighbourhoods.

He said troops fired in the air to disperse the protesters. Activists also said troops opened fire at protesters in Aleppo, Syria's largest city, as well as the central city of Hama. They had no immediate word on casualties.

In the rebel-held Khaldiyeh neighbourhood in the city of Homs, which has become the heart of the uprising, a mortar round was striking every five minutes, according to the British-based Syrian Observatory for Human Rights. An amateur video posted online by activists showed thick black smoke billowing as shells fell in a residential area.

The Observatory said eight civilians were killed in Homs on Friday, including a family of three whose home was struck by a shell. The group reported three more civilians were killed by army fire in other parts of Syria.

Citing its network of sources on the ground, the group said explosions and the crackle of gunfire rang out in the town of Qusair, near the border with Lebanon. Activists said the government was sending reinforcements to the town.

"Regime forces are fortifying their positions in eastern and western Qusair," about seven miles from Lebanon, said the head of the Observatory, Rami Abdul-Rahman.

Meanwhile, Syria's state-run news agency Sana said a large roadside bomb went off in the southern village of Sahm al-Golan, near the Israeli-occupied Golan Heights, killing 10 soldiers. It gave no further details.

In Paris, France's foreign minister, Alain Juppé, said on Friday that the international community had to live up to its responsibilities in Syria and prepare for the possible failure of an increasingly fragile ceasefire. He told France's BFM television that if special envoy Kofi Annan's peace plan "doesn't function, we have to envisage other methods".

Clinton referred during the Paris meeting to a resolution under the UN charter that would be militarily enforceable.

"We need to start moving very vigorously in the security council for a chapter seven sanctions resolution, including travel, financial sanctions, an arms embargo, and the pressure that that will give us on the regime to push for compliance with Kofi Annan's six-point plan," she said.

The Syrian opposition welcomed her comments.

"The fact that Mrs Clinton talked about this resolution shows that the international community is preparing to take stronger action against this cruel regime," said Fawaz Zakri, an Istanbul-based member of the Syrian National Council.

Any attempt to push for UN sanctions would be likely to meet resistance from Syria's allies Russia and China, which hold vetoes in the security council. Moscow and Beijing have already twice shielded Syria from UN sanctions over Assad's crackdown on a popular uprising that is estimated to have left 9,000 people dead and led refugees to pour into neighbouring countries.

Ban has recommended that the security council quickly approve a 300-member UN observer mission to Syria, a number larger than was originally envisaged. But he said he would review ground developments before deciding when to deploy the mission.

Ahmad Fawzi, a spokesman for Annan, said in Geneva on Friday that the UN hoped to have 30 ceasefire monitors in the country next week.

An amateur video posted online by activists showed the head of the observers' advance team, Colonel Ahmed Himiche, talking to residents in the southern town of Khirbet Ghazaleh on Thursday. Himiche asked them whether schools and hospitals were available in the town.

"They [troops] assassinate whoever we take to the general hospital," one man replied. AP Beirut